Calculating SAC?

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stuartv

Seeking the Light
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I got my Computer Nitrox certification last week. During the class, the instructor talked about figuring out your SAC.

He said to go down to some depth, note your SPG reading, stay at that depth for 10 minutes, then note your SPG again. Using those numbers, you can calculate your SAC. And I understand that (the physics/math).

But, it seems like the number you come to would never match the real world of actually breathing while swimming around on a dive. Even if you're mostly just floating or drifting.

I mean, if you use that number, and you assume a square dive profile, and you calculate your BT so that you should arrive back at the surface with, say for example, 500 psi, and then you dive that profile, as close as you can, I would expect you to always hit the surface again with less than 500psi. Maybe even by a significant amount.

It SEEMS obvious to me that if you use an air-integrated computer and download the data afterwards, you'd be able to have software tell you what your real SAC was. I.e. give you an SAC that is accurate enough to plan a similar dive in the future and get out with pretty close to the predicted amount of air (assuming similar profile and conditions).

But, since I know you more experienced folks, and especially the tech diver folks, mostly eschew air integration, how do you really figure out an accurate SAC for yourselves?

As best I can figure, it seems like you'd start with a baseline number, as my instructor described, then use that to predict results. And when your results were off, you'd adjust your baseline number accordingly, so that, as you repeat the process, you gradually work out a SAC number for yourself that works to give you somewhat reasonably accurate predictions of actual consumption.

Example: You follow the instructions and calculate that you have a .9 l/min SAC. You plan a dive based on that that should give you 45 minutes of BT and put you back on the surface with 500psi left. You dive your plan and end up having to ascend after only 38 minutes, arriving back at the surface with 500psi.

Based on that, you revise your SAC from .9 to (for discussion purposes, 0.9 * 45/38 =) 1.1 l/min. So, you plan your next dive based on an expected SAC of 1.1, instead of .9.

Is that more or less how it goes? Or is there a better or easier way?

Or do you just take the number from sitting at a fixed depth for 10 minutes and go with that because it's close enough?

Thanks!
 
Your instructor's method seems reasonable. However, you could make your instructor's method more accurate by swimming around at that fixed depth for the 10 minutes. The method I was taught goes even further than that.

I was taught to compute SAC for each actual dive by using my average depth (estimated, since my computer doesn't compute average depth) for the entire dive, the amount of gas consumed over the entire dive, and the number of minutes of bottom time for the dive. Gas consumed is computed by subtracting the SPG reading after surfacing from the SPG reading before descending, then converting to pressure to volume. The resulting SAC for that dive then becomes one of a number of data points that are averaged together over some number of dives. The idea is that SAC will vary depending on the dive profile, etc., so an average is a more useful number.
 
Thanks, Lorenzoid. That clears it up some, for sure. I have read several times about references to computers that report average depth, and I have the impression people use that for calculating repetitive dives. I had not thought about the possibility of using it for calculating SAC. I haven't thought about it much at all, really, since I don't think any of the PDCs I've been looking at even have average depth.
 
Based on that, you revise your SAC from .9 to (for discussion purposes, 0.9 * 45/38 =) 1.1 l/min. So, you plan your next dive based on an expected SAC of 1.1, instead of .9.

Just a clarification. When you calculate your SAC using your SPG the resulting calculation will be in psi/min (pressure per unit time). You can then convert to ft3/min (volume per unit time) knowing the starting cylinder (gas) volume. You specify 0.9 or 1.1 l/min (liters/min) but I think you meant 0.9 or 1.0 ft3/min. The volume per unit time is also called RMV (residual minute volume). A SAC of 40 psi/min will give you 1.0 ft3/min (28 l/min) using an AL80 tank. Be careful. Keep your units consistent.
 
Mine does not show Avg Depth until I down load it to my computer - it only shows me max depth, current depth and dive time. But when I connect it to a PC it has lots of information that is not shown on the actual PDC. I am one of those that uses my PDC to track my dives and RMV/SAC rate - you can do the same with Excel.

SAC/RMV = ((PSI Used / Working Pressure) x Cylinder Capacity) / (((Depth / 33) + 1) x Minutes)
PSI used is from start to finish - example 3000 PSI start 1800 PSI end = 1200 PSI used
Working pressure is generally 3000 PSI for AL80's
Most are AL80's or 77.4 cu ft
Depth is your avg depth - but (depth/33)+1 gives you the pressure
Minutes is the overall time from start to finish

I use an excel sheet to track my SAC/RMV times so I can understand what was different about each dive - my log does that for me too but not all in one place where I can look for trends...

The advantage is that you can use another formula for planning dives -
Volume = SAC x Pressure x Time
And this can tell you how many cubic feet of gas you will need at depths and times.

*** added
to your point Stuartv - one SAC/RMV rate is next to meaningless but if you track over a month or a season of dives it becomes more meaningful - at least to me. I dont do the same dives in the same month or season and so I can look to see - was I nervous and that is why my SAC was high? Was it my first time at that site and it had a heavy current? Things like that make it more meaningful... in my opinion and so I use a Log to make notes of the dives as well so I can review.
 
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Just a clarification. When you calculate your SAC using your SPG the resulting calculation will be in psi/min (pressure per unit time). You can then convert to ft3/min (volume per unit time) knowing the starting cylinder (gas) volume. You specify 0.9 or 1.1 l/min (liters/min) but I think you meant 0.9 or 1.0 ft3/min. The volume per unit time is also called RMV (residual minute volume). A SAC of 40 psi/min will give you 1.0 ft3/min (28 l/min) using an AL80 tank. Be careful. Keep your units consistent.

I was pulling numbers out of the air for the sake of understanding the process.

---------- Post added November 18th, 2014 at 04:06 PM ----------

Mine does not show Avg Depth until I down load it to my computer - it only shows me max depth, current depth and dive time. But when I connect it to a PC it has lots of information that is not shown on the actual PDC. I am one of those that uses my PDC to track my dives and RMV/SAC rate - you can do the same with Excel.

SAC/RMV = ((PSI Used / Working Pressure) x Cylinder Capacity) / (((Depth / 33) + 1) x Minutes)
PSI used is from start to finish - example 3000 PSI start 1800 PSI end = 1200 PSI used
Working pressure is generally 3000 PSI for AL80's
Most are AL80's or 77.4 cu ft
Depth is your avg depth - but (depth/33)+1 gives you the pressure
Minutes is the overall time from start to finish

I use an excel sheet to track my SAC/RMV times so I can understand what was different about each dive - my log does that for me too but not all in one place where I can look for trends...

The advantage is that you can use another formula for planning dives -
Volume = SAC x Pressure x Time
And this can tell you how many cubic feet of gas you will need at depths and times.

Got it. Makes perfect sense! Thanks!
 
If you can find it you should pick up Steve Lewis' book "The Six Skills and Other Discussions: Creative Solutions for Technical Divers". In spite of the title is is also an excellent book for new divers who are interested in advancing. In the book he discusses SAC rate in the context of gas planning and advocates applying a factor that will account for potential stressors on the dive e.g. unfamiliar site, wreck penetration, poor visibility or cold water. Armed with this information, you can add a safety factor to your calculations which includes heavier than normal breathing. I have the book and have read it cover to cover several times and learn something new every time.[h=1][/h]
 
Just a clarification. When you calculate your SAC using your SPG the resulting calculation will be in psi/min (pressure per unit time). You can then convert to ft3/min (volume per unit time) knowing the starting cylinder (gas) volume. You specify 0.9 or 1.1 l/min (liters/min) but I think you meant 0.9 or 1.0 ft3/min. The volume per unit time is also called RMV (residual minute volume). A SAC of 40 psi/min will give you 1.0 ft3/min (28 l/min) using an AL80 tank. Be careful. Keep your units consistent.

Even though this is completely correct, I am sure you're probably lost. The official SAC rate is a number reported in PSI and changes based on what tank you diving...a low pressure changes at a different rate than a high pressure tank. 100 psi =3.788 cuft of air in a low pressure tank, but 100 psi = 2.905 cuft in a high pressure tank. The RMV is a number that is reported in cubic feet per minute. Many divers, myself included, use them interchangeably. To calculate a SAC/RMV you need a few parts.

A) Total Air Used
B) Depth (I call it the pressure multiplier)
C) Time
D) SAC RATE/RMV

A (Total Air Used) = B x C x D

Example:
Lets say I did a dive with a full 100 cuft (starting pressure 3442) tank for 35 minutes with an average depth of 40ft and ended with 1300 psi.

Total Air Used = 2142/3442 = 62.2 cuft
Pressure multiplier = (40+33)/33= 2.212
Time= 35 minutes (since I am keeping my SAC/RMV in minutes I don't need to change anything here in terms of units--if it was 1.5 hours, you would need to convert to minutes)
SAC= ?


62.2 = 2.212*35*X

X= .803 cuft a minute is your RMV

So you use .803 cuft of air a minute on the surface. To convert this is PSI you would need to take .803/size of tank * working pressure of tank (.803/100 * 3442) which would get you a SAC rate of 27.6 PSI a minute. You can see how this will change if you used a low pressure 95 tank (.803/95 * 2640) and it would give you a SAC rate of 22.3 PSI for a LP95.

As you can see it is kind of annoying to look at the PSI for different tanks, so many people say how much air they use in a minute (.5, .6 cuft/min, etc). Where the difference between SAC and RMV got confused I don't know, maybe SAC is just cooler and easier to say. Surface Air Consumption is definitely more intuitive when speaking rather than saying Respiratory Minute Volume.

As for you other question about SAC rate and dive planning. SAC rates are used as general guide and can change based on task loading, the temperature, how in shape a diver is. Example I had a SAC rate of .55 a few months ago, I gained 10 pounds and wasn't running as much and my SAC rate is closer to .61 now. Whenever you are planning a dive, you use it as a baseline and add some conservative factors in terms of depth, time, and/or rate.

If I was planning a multi level dive, I would use a .65 or .7 SAC rate and then make my depths and time at them pretty conservative. Maybe I know I will swim along this ridge that is 50 ft until I get to X. I might estimate it will take 10 minutes to do it when I am calculating how much air I will need, but I know in reality it will probably be around 6-7 minutes in real life. There might be an attraction at ~75 ft, but I will use 80 ft when calculating how much of a pressure multiplier to use. Every dive is different, it is a matter of adding up factors and creating margins of safety.

I planned an elaborate quarry dive with my dive partner. We were going to explore a whole bunch of attractions and use some navigational markers. I gave us both .6 SAC rates, and estimated time and depth it would take to get to each attraction. I calculated that we would need 92 cuft of air. I also wanted to end at no less than 400PSI. I was diving a HP100, so that 92 cuft of air that estimated would have left me with 275 PSI in my tank. I was very conservative with my times (longer times) and depths (deeper depths) so I knew I would probably be fine with the 100. but I still told my buddy that we would need to use his 108's for this dive. The 108's gave me an added safety margin and allowed me to end the dive at 400 PSI (we actually ended at 750 PSI, but again, conservative is the name of the game)
 
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Even though this is completely correct, I am sure your are probably lost. The official SAC rate is a number reported in PSI and changes based on what tank you diving

I gather you are talking to me? I spent two years majoring in Mechanical Engineering in college before I switched to Applied Math. I was definitely not lost.

But, I really do appreciate you taking the time to write that post. I would also go out on a limb, though, and say that no "rate" can be reported in PSI. However, it could be reported in PSI per minute. Or liters per minute. Moles per second. Pounds per hour. Stone per fortnight. Or any other units you want that are suitable for the measurements being taken. :-D
 
Hi Stuart,

I use the metric system as its so much simpler.

SAC = (Lt used)/((Ambient Pressure in Bar)+1)/(Dive time)

Lt used is the start pressure minus the end pressure (in Bar) multiplied by the cylinder size (in Lt).
Ambient Pressure is taken from average depth (in meters) and dividing by 10.
Dive time is in minutes.

For today's dive ((300-100)*12)/((13.1/10)+1)/67 = 15.51Lt/Min
 

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